The term “bimodal” or “multimodal” as applied to polyolefin resins usually means that the resin has at least two distinct ranges of molecular weight that may impart desired properties to the product in great variety. Bimodal resins were typically made in two separate reactors connected in series, for example, a product having a first molecular weight was moved directly from a first reaction zone in which it was made and introduced to a second reaction zone usually providing different polymerization conditions for making a polymer composition. Two-stage processes are difficult to control and, perhaps more important, have a capital disadvantage. Moreover, frequently the polymer products are not homogeneously mixed in that at least some particles are entirely of one modality or the other. It is therefore desirable to find ways of making homogeneous bimodal polyolefins in a single reactor.
Alternatively, one approach to making bimodal polyolefin compositions in a single reactor has been to employ a mixed catalyst system, in which one catalyst component makes a primarily low molecular weight (LMW) product and the other catalyst component produces a primarily high molecular weight (HMW). For example, bimodal catalysts are often used to co-polymerize polymers having two average molecular weights using a single catalyst system. By including both of these catalyst components in the same catalyst system, a bimodal product can be produced. The polymer having different molecular weights are mixed at the molecular level providing a polymer product that is relatively free of gels compared to similar products made in staged-reactor or series-reactor processes or by the blending of two distinct unimodal resins.
Controlling the ratio of the components in the bimodal polymer product or composition is a significant manufacturing concern. Product properties of bimodal resins are often sensitive to component split. For instance, in the manufacture of high-density, high-molecular-weight film, to achieve the desired specification may require control of component split within about 2% of the setpoint.
The weight percentage or “split” of the HMW and LMW in the total polymer product is greatly influenced by the relative amount of each type of catalyst in the catalyst system. While theoretically, a catalyst system containing proper amounts of each catalyst could be generated and used to produce the desired split in a particular case, in practice using such a system would be difficult, as the relative productivities of the catalyst components can change with variations in reactor conditions or poison levels.
A technique for changing the flow properties of a bimodal resin is by changing the resin component split, or weight fraction of the HMW component in the product. By modifying the relative amounts of HMW and LMW components in the resin, flow properties can be changed as well. Unfortunately, in some cases changing the split affects more than one variable. In some products, changing the HMW split by a few percent can significantly affect both resin flow index and Melt Flow Ratio (MFR).
MFR is a ratio of two different melt flow index measurements, and is used to quantify the shear-thinning of the polymer. As is well known, melt flow index measurements measure the rate of extrusion of thermoplastics through an orifice at a prescribed temperature and load, and are often used as a means to discern molecular weight of the overall polymer.
Generally, it has been believed in the art that reducing hydrogen concentration during polymerization using a bimodal catalyst system would increase product MFR by increasing the spread of the HMW and LMW product components.